1. Field of the Invention
The subject invention relates to valves and, more particularly, to control valves for metering additives to a fluid flow at a precisely controlled rate.
2. Description of the Prior Art
In the prior art, there has existed a need for a device that would provide an additive to a fluid flow in highly dilute concentration. For example, natural gas is generally distributed in pipeline systems wherein the gas is transmitted at pressures ranging between 100 and 3,400 lbs. Before the gas reaches the local distribution network, where the pressure can be as low as four to six ounces, alcohol must be added in amounts of about 60 parts per million to prevent concentration of entrained water vapor resulting in rusting of the gas burners. Additional additives are also combined with the gas in highly dilute concentration for other purposes. For example, mercaptian is provided in a proportion of about 3.7 parts per million to give the gas a detectable odor.
Regulated control valves known in the prior art are generally unsuited for use in applications when an additive is to be added to a fluid flow in dilute concentrations. U.S. Pat. Nos. 2,243,454 and 3,353,550 are generally illustrative. In the prior art, devices for metering additives into a fluid flow have generally provided these additives at a constant rate which is unregulated with respect to the rate of fluid flow. For example, in the gas industry, alcohol is generally metered from an injection tank into the transmission line through an ordinary globe valve. The position of the valve is set by an operator. The alcohol injection points are usually at remote locations where the operator is only infrequently in attendance. Accordingly, although the flow rate of gas in the transmission line may change or be cut off entirely, the alcohol continues to be metered through the globe valve at the initial rate until the operator returns, perhaps days or even weeks later. Consequently, considerable quantities of alcohol are wasted.
Other devices known in the prior art did not control the flow of the additives at all. Rather they operated merely in either open or closed modes in which the additive was allowed to flow freely or was stopped entirely. An example is shown in U.S. Pat. No. 3,905,451.
Although some mechanisms were known in the prior art that would regulate the flow of additives into a fluid stream, they provided insufficient regulation for many applications-particularly where the additives are in highly dilute concentrations. For example, U.S. Pat. No. 1,937,893 is directed to a device for providing rust inhibitors to a boiler water supply. As described therein, a vertically movable self-seating valve in the water supply line controls a ball valve that determines the flow of the rust inhibitors. However, the self-seating valve has a vertical freedom of movement such that due to line pressure fluctuation and turbulance in the water flow, the vertical position of the self-seating valve is not stable and precise regulation does not occur. Other examples are shown in U.S. Pat. Nos. 895,709 and 926,848 wherein carburetors are disclosed in which needle or drum type valves are responsive to the position of a suction valve. Although in U.S. Pat. No. 926,848 movement of the suction valve is damped, there is still insufficient regulation of the additive for use in applications where there are highly dilute concentrations.